In an all-solid Li-ion secondary battery, solid electrolyte is used. Therefore, the all-solid Li-ion secondary battery has high thermal stability compared with a Li-ion secondary battery in which nonaqueous electrolyte is used. Among others, an oxide solid electrolyte has the feature of also being chemically stable. Main characteristics required from a solid electrolyte include, e.g., high lithium ion conductivity, good chemical stability, and a wide potential window. Among those characteristics, Li7La3Zr2O12 (hereafter simply referred to as “LLZ”) satisfies the chemical stability and wide potential window requirements. For this reason, LLZ is considered one of the most promising solid electrolyte candidates (see, for example, Japanese Patent No. 5132639 and Angew. Chem. Int. Ed., 2007, 46, 7778-7781).